add support to dumps for the default arg, like json.dumps...

pasn1.py

@@ -15,7 +15,7 @@ __author__ = 'John-Mark Gurney'
 __copyright__ = 'Copyright 2016 John-Mark Gurney.  All rights reserved.'
 __license__ = '2-clause BSD license'
 
-# Copyright 2016, John-Mark Gurney
+# Copyright 2016-2019, John-Mark Gurney
 # All rights reserved.
 # 
 # Redistribution and use in source and binary forms, with or without
@@ -321,12 +321,24 @@ class ASN1Coder(object):
 
 		return r, end
 
-	def dumps(self, obj):
-		'''Convert obj into an array of bytes.'''
+	def dumps(self, obj, default=None):
+		'''Convert obj into an array of bytes.
+
+		``default(obj)`` is a function that should return a
+		    serializable version of obj or raise TypeError. The
+		    default simply raises TypeError.
+		'''
+
 
 		try:
 			tf = self._typemap[type(obj)]
 		except KeyError:
+			if default is not None:
+				try:
+					return self.dumps(default(obj))
+				except TypeError:
+					pass
+
 			if self.coerce is None:
 				raise TypeError('unhandled object: %s' % `obj`)
 
@@ -645,3 +657,19 @@ class TestCode(unittest.TestCase):
 		'''Verify that ASN1Coder does not support dict.'''
 
 		self.assertRaises(KeyError, ASN1Coder().loads, dumps({}))
+
+	def test_dumps_default(self):
+		'''Test that dumps supports the default method, and that
+		it works.'''
+
+		class Dummy(object):
+			def somefun(self):
+				return 5
+
+		def deffun(obj):
+			try:
+				return obj.somefun()
+			except Exception:
+				raise TypeError
+
+		self.assertEqual(dumps(5), dumps(Dummy(), default=deffun))